1. Field of the Invention
This invention relates generally to spectral equalization circuits and more particularly to an equalizer for magnetic recording and signal processing applications in which the spectral contour is automatically adjustable over a wide range to compensate for component and recording track tolerances and to enhance signal resolution.
2. Description of the Prior Art
It is well known that when signals conveying information are transmitted through a channel they undergo amplitude and phase distortion, often called spectral distortion. To the extent that the transmission channel may be characterized as a linear network having a given frequency transfer function, this spectral distortion may be compensated for using well known signal processing circuits called equalizers. Generally the equalizer is used to negate the effects of the channel as characterized by the given frequency transfer function.
As an example of a prior art equalizer, reference may be made to Kameyama et al, "Improvement of Recording Density by Means of Cosine Equalizer", IEEE Transactions on Magnetics, Vol. MAG-12 No. 6, Nov. 1976. This prior art equalizer has the disadvantage of not being automatically adjustable in response to an external stimulus, such as temperature or pressure, nor can it readily compensate for manufacturing tolerances or track width related storage density variations.
In the magnetic recording art, for instance, an information bearing signal in the form of a write current is passed through a write head which induces proportionally varying degrees of magnetism in a magnetic medium moving past the head. The magnetic medium retains its magnetization which constitutes a recording. Playback is the process in reverse: the magnetization in the medium induces varying electric currents in a read head and the currents are transformed into a reproduction of the original signal. As is generally true with infomation bearing signals transmitted through a channel, the signals derived from the read head undergo a certain amplitude and phase distortion. This distortion manifests itself in such phenomena as peak shift and amplitude loss which can degrade the data recovery reliability. One cause of this distortion is intersymbol interference or the interraction between adjacent bits of information. Intersymbol interference is becoming increasingly troublesome as bit storage densities are pushed higher and higher.
Related to the problem of providing reliable data recovery at high storage densities is the costly problem of fabricating magnetic recording heads to very close tolerances. For example, a typical magnetic recording system might employ a read head having a gap of 55.mu. in and operating at a flying height of 20.mu. in. These dimensions have a direct bearing on the resolution of the system and thus must be maintained to very close tolerances. This is especially true in fixed head systems, for example, which might typically employ 24 heads on each side of a two sided magnetic disc medium. In such a system considerable effort must be expended to assure that all of the heads have practically the same resolution and response characteristics. The problem is compounded when the system must work under environmental extremes, such as at very low and very high temperatures. At high temperatures, for instance, much greater peak shift is exhibited due to temperature induced physical changes in head and medium. As an example, high temperature affects the aerodynamic pressures around the head, causing increased flying height and degraded read back signal resolution.
In prior art practice, when these environmental extremes were encountered the recording system had to be designed on a worst case basis. That is, the system would typically be a compromise between optimum high temperature performance and optimum low temperature performance. For instance, at high temperatures it is desirable to equalize more severely than at low temperatures so as to narrow signal pulses. However, to do so would result in over equalization at room temperatures which causes ripples in the waveform and severe noise increase.
Thus there is a need for a means to overcome the problem of spectral distortion encountered in information systems which operate under extreme environmental conditions or with very precise manufacturing tolerances.
Another problem common to disc media is that data storage density is not uniform across the disc. The outermost track of a given circumference can store data with less crowding than does the innermost track, which has a much shorter circumference. Thus the track radius is another factor which can affect data recovery.